1. Field of the Invention
This invention relates to direction finding (DF) apparatus for use with directional antennas pointing at a multiplicity of different angles. Such DF apparatus typically detects a localized source of radio-frequency radiation using antennas and receivers suited to the particular waveband of interest, and the antenna boresights are typically coplanar, but the invention is applicable to other DF systems.
2. Description of Related Art
Known DF apparatus comprises several antennas, with coplanar boresight axes coinciding at a point, the antennas spaced equi-angularly to obtain 360.degree. cover in one plane. The angular separation is typically equal to the 3 dB beamwidth of each antenna, i.e. the angle bounding a sector within which the power received by the antenna is within 3 dB of its maximum.
Such conventional apparatus locates a radiation source by first determining the angular sector in which the received signal is strongest, and then comparing the received powers from the two whose boresights bound that angular sector, in order to determine the angular position of the source within that sector. The expected variation of this power ratio with source angle is a known quantity, and it is usually possible to locate a source to an accuracy of one tenth of an angular beamwidth or sector width.
It is not normally possible to increase the angular separation of the antennas much beyond the beamwidth because this increases the dynamic range requirements of the receiver and requires exceptional control of the antenna pattern. It follows that in order to achieve a better DF accuracy over a given field of view more receiver channels would have to be used and the cost of the overall receiver would increase rapidly.